Now that you've got the right sizes of both plastic and paper, you can turn on your iron. The right iron heat should be a tad little bit less than the maximum. Put the paper over the plastic sheet and let them fuse. The iron shouldn't come into direct contact with the plastic. If you need to iron the borders, cover the plastic with some other paper. You can use baking paper if you don't want them to stick.
When you iron the paper onto the plastic, the plastic will shrink and then burn onto the paper and then shrink again. This means your paper will get a bit crumpled, but the good thing is it will get all even again if you iron it onto the piece of clothing.